The present invention is a layered electrophotographic photoconductor, i.e., a photoconductor having a metal ground plane member on which a charge generation layer and a charge transport layer are coated, in that order. Such a photoconductor may optionally include a barrier layer located between the metal ground plane member and the charge generation layer, and/or an adhesion promoting layer located between the barrier layer and the charge generation layer, and/or an overcoat layer on the top surface of the charge transport layer. In photoconductors of this type, the charge generation function and the charge transport function are provided by different discrete layers that are coated at different times during the manufacture of the photoconductor.
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between areas on the surface exposed and unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges on the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, in the preferred embodiment the photoconductor and toner have the same polarity but different levels of charge.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers melts and fixes the toner in the paper, subsequent to direct transfer, or indirect transfer when an intermediate transfer medium is used, producing the printed image.
The electrostatic printing process, therefore, comprises an ongoing series of steps wherein the photoconductor surface is charged and discharged as printing takes place. It is important to keep the charge voltage and discharge voltage on the surface of the photoconductor constant as different pages are printed in order to make sure that the quality of the images produced are uniform (cycling stability). If the charge/discharge voltages change each time the drum is cycled, e.g., if there is fatigue in the photoconductor surface, the quality of the pages printed will not be uniform and will be unsatisfactory.
It is desirable to use liquid toners in the electrophotographic printing process in order to get the highest possible resolution on the printed page. However, on most photoconductive surfaces, the oil carrier present in the liquid toner tends to extract charge transport molecules from the photoconductor drum. This destroys the toner and results in higher discharge voltages on the drum and poor quality in the printed pages produced. This oil extraction, which results from the use of liquid toner, may also disrupt the charge generation layer.
Thus, it is important, when designing a photoconductor, to have one which maximizes both oil resistance when liquid toners are used, and cycling stability.
The present invention, by using specific components in the charge generation and charge transport layers, provides both improved cycling stability and improved oil resistance when compared to conventional photoconductors. The present invention utilizes phthalocyanine dyes, preferably X-form metal-free phthalocyanines, together with a medium molecular weight polyvinyl chloride binder in the charge generation layer, and a polyestercarbonate binder in the charge transport layer to provide these improved results.
X-form metal-free phthalocyanine materials have been disclosed for use as pigments in photoconductors used in electrophotographic reproduction devices. U.S. Pat. No. 5,168,022, Wasmund et al., issued Dec. 1, 1992, describes a process for making X-form phthalocyanine materials. These materials are taught to be useful in the charge generation layer of a photoconductive imaging member together with polymeric binders, such as polyvinylbutyral and polyvinylacetate.
U.S. Pat. No. 3,816,118, Byrne, issued Jun. 11, 1974, describes electrophotographic plates which include a phthalocyanine pigment dispersed in a binder. X-form phthalocyanine is preferred. Binders useful in the disclosed invention include polyvinyl chloride and polycarbonate, as well as many other conventional binder materials.
U.S. Pat. No. Reissue 27,117, Byrne et al., reissued Apr. 20, 1971, discloses X-form phthalocyanine which is taught to be useful as a photoconductive material when mixed with a binder and coated on a substrate.
U.S. Pat. No. 5,364,727, Nguyen, issued Nov. 15, 1994, describes a positive charging photoconductor, for use with a liquid toner, comprising a fine particle phthalocyanine pigment and an amine-type sensitizer distributed in a polymeric binder. X-form phthalocyanines are taught to be preferred pigments, while any conventional polymeric binder, including polycarbonates, are taught as being useful.
U.S. Pat. No. 5,075,189, Ichino, issued Dec. 24, 1991, describes an electrophotographic photoreceptor which comprises an N-alkoxylated or N-alkylated polyamide copolymer undercoat layer together with a charge generating layer which includes a pigment and a conventional binder resin. X-form phthalocyanine is taught to be one of many pigments with which may be used in the disclosed invention, together with conventional binders including vinyl chloride resin and polycarbonate resin.
U.S. Pat. No. 5,204,200, Kobata, et al., issued Apr. 20, 1993, describes a laminated organic photosensitive material which utilizes an alcohol-soluble polyamide resin as an undercoat. In this invention, the charge generating layer contains an X-form phthalocyanine and, as a binder, a mixture of vinyl chloride-ethylene copolymer and vinyl chloride-vinyl acetate-maleic acid copolymer. The binder resin in the charge transport layer may be a polycarbonate material.
U.S. Pat. No. 4,218,528, Shimada, et al., issued Aug. 19, 1980, describes a method for forming an electrostatic image using an activation light to prevent dark decay. In this method, the photoconductor includes a fine photoconductive powder, such as metal-free phthalocyanine among many others, dispersed in a resin which may include polycarbonates, although phenol resins are preferred.
U.S. Pat. No. 4,973,536, Horie, et al., issued Nov. 27, 1990, describes an electrophotographic photoreceptor which includes a phthalocyanine pigment in the charge generating layer and a specifically-defined hydrazone in the charge transport layer. X-form phthalocyanine is specifically disclosed, although not preferred. Useful binders include polycarbonates and polyvinyl chloride, among many others.
U.S. Pat. No. 4,975,350, Fujimaki, et al., issued Dec. 4, 1990, describes a photoreceptor which includes X-form metal-free phthalocyanine in the charge generating layer together with conventional binders, including polycarbonates and polyvinyl chloride. The disclosed photoreceptor also includes specifically-defined carrier transport materials, including hydrazones.
U.S. Pat. No. 5,053,303, Sakaguchi, et al., issued Oct. 1, 1991, describes electrophotographic photosensitive materials which comprise X-form metal-free phthalocyanine and a binder (polyvinyl chloride is not disclosed) in the charge generating layer, and a hydrazone, butadiene or pyrazoline compound together with a polycarbonate binder in the charge transport layer.
Polyvinyl chloride and polycarbonates are also known as binders in photoconductor structures. For example, U.S. Pat. No. 5,130,215, Adley, et al., issued Jul. 14, 1992, describes a layered photoconductor which utilizes a specific ordered polyestercarbonate as a binder in one or both of the charge transport and charge generating layers. A structure having a charge transport layer comprising a hydrazone and a polyestercarbonate binder is specifically disclosed. It is taught that the invention exhibits reduced discharge area fatigue, as well as reduced fatigue upon exposure to room light. See also U.S. Pat. Nos. 4,973,536; 3,816,118; 5,364,727; 5,204,200; 4,975,350; and 5,053,303, all of which are discussed above, regarding the use of polyvinyl chloride and/or polycarbonates in photoconductors. However, none of these patents-disclose the specific combination of X-form metal-free phthalocyanine and medium molecular weight polyvinyl chloride binder in the charge generating layer, and polyestercarbonate binder in the charge transport layer which are required to achieve the benefits of the present invention.